Thermocouples are widely used for temperature measurements of various gases and liquids. If bare thermo-element wires are exposed directly to detrimental atmospheres and fluid, they are often physically and chemically affected resulting in reducing service life with severe deterioration and corrosion. Thermocouples are, therefore, usually protected with insulators and protection tubes. In selection of suitable insulators and protection tubes, consideration should be given to the materials especially of heat resistance, mechanical strength, chemical stability, etc. depending on the respective operating conditions. This is the most important point in thermometric practice.

Thermowells which are made of solid bar stock of various heat and corrosion resistant alloys by drilling are usually preferred over the tip welded protection tubes for critical applications where high mechanical strength and longer service life are required. If the alloy bar material is correctly selected and designed properly, the Thermowell lasts long against corrosives, high pressure, high temperature, mechanical shock and vibration that may result from high velocity of fluids. In order to offer the best and safest. A specially developed computer programme as based upon operating conditions at the site makes Thermowells against Ka’rma’n’s Turbulence and other stresses, automatic calculations of mechanical strength to fluid pressure and flow velocity to estimate frequency of critical resonance. At TECHNO, a genuine Two Shaft Gun Drilling Machine of two-metre max manufactures Thermowells.

Thermowells are ideal where a sensor is required to be inserted into a process where external elements such as pressure, corrosion, or abrasion affect the life of the sensor. . Techno has been manufacturing solid barstock type Thermowells to accommodate applications in the petrochemical, chemical, refining, power and other process industries for many years. Threaded, flanged, weld-in, socket, Van Stone and other styles are custom made to your specs or available from our large inventory for immediate shipment. Optional coatings and sprays are also offered.

Material of Construction

Thermowell material must be chemically compatible with the process system and the temperature sensor. In most cases, thermowell selection is based on the corrosive conditions in the well environment. Sometimes The selection may be based solely on the mechanical strength needed to withstand operating pressure and process flow. Often a combination of factors must be considered. In addition to selecting the proper base material, coatings may be used to improve a thermowell’s resistance to abrasion or the chemical process.

The thermowell wall must be thin enough to minimize sensor error caused by thermal conduction and slow sensor response, but thick enough to withstand collapse from process pressure, erosion from abrasive media and bending from the process flow. Spring-load mounting styles are recommended to ensure positive contact to maximize thermal transfer and minimize sensor vibration within a thermowell.

Insertion Length

The insertion length or ‘U’ length is the distance from the end of the well to the underside of the thermowell thread or other connection device. For maximum accuracy, this length must be long enough to permit the temperature sensor to be fully immersed in the media to be measured and minimize sensor error caused by thermal conduction, But short enough to withstand damage caused by process flow vibration. As a general rule of thumb, the thermowell should extend into the process a minimum of 10 times the sensor diameter or, in the case of RTDs, 10 times the sensor diameter plus one inch. This should extend the sensor into the process between 1/3 and 1/2 the diameter of the process pipe. The insertion length must also take into consideration any dead length required to pass through walls, pipe fittings and insulation.

Velocity

The most common cause of well failure is the vibration effect caused by fluid forming a turbulent wake as it flows past the well. This turbulence has a definite vibration frequency based on the diameter of the well and the velocity of the fluid. The well must have sufficient stiffness to ensure that the wake frequency will never equal the natural frequency of the well. If the natural frequency of the well coincides with the wake frequency, the well will potentially vibrate to destruction. To be in compliance with the ASME Performance Test Code, the thermowell should have a natural frequency a minimum of 125% of the wake frequency. Tapered shank wells (heavy duty – Type H) have a high strength-to-weight ratio with a resultant higher natural resonant frequency than the equivalent length straight shank well. Tapered shank wells are preferred for operation at higher fluid velocities.

Process Connection

Techno Instruments Technologies provides standardized wells in most of the common connection types, including threaded, flanged and socket weld types with standard bore sizes. Threaded wells are available in materials that can be readily welded. Flanged wells are manufactured by welding a bar stock well to the specified flange style. Doubled-welded construction reduces crevice corrosion and stress problems by ensuring that no open joints are exposed inside or outside the installation.

Bore Size

Selection of a standard bore size throughout the plant permits the use of several types of temperature measuring instruments in the same wells. Techno standard bore sizes fit most commonly used temperature sensing devices. Most applications use 0.260″ or 0.385″ diameter bores. This number represents the inside diameter of the well, expressed in thousandths of an inch.

Protective coatings

For some applications, thermowell resistance to environments can be greatly improved with surface applied coatings. Thus, a thermowell exposed to abrasion may be provided with a hard surface by applying suitable alloy coatings to the outside. While hardness levels tend to fall off with increasing temperature, some of these coatings remain quite hard (e.g. Rockwell C45) up to at least 1000 Deg F. Techno is market leader in such coated Thermowells and had provided hundreds of its clients with positive drift in their cost-efficiency curve with the help of such protective coatings.

Typical Applications
Cement Plants	Outlet Ducts
Refineries	Chemical Reactors
High Speed Mixtures	Boiler Beds
Coal mills
Available Coatings

Tungsten-Carbide (Co-matrix)	Teflon type
Aluminum Oxide	Rubber Coatings
Zirconium Oxide	Hastalloy Sleeves
Monel Sleeve	Boron carbides
Titanium Sleeve	Tantalum Sleeve
FEP Sleeve	Glass lining
Titanium-Oxide Coating	Nickel Sleeve
Chromium carbide	Others.

Thermowell Material Specifications	Thermowell Materials Selection Guide